As a result of the increasing interest in renewable resources in general and biofuels in particular, a number of processes has been developed for the production of esters of fatty acids and lower alcohols, which esters are also referred to as ‘biodiesel’. Early ‘biodiesel’ processes prescribed the use of neutral raw materials and thereby competed with food applications. Accordingly, there is an incentive to exploit cheaper alternative sources of fatty acid moieties as raw material for biodiesel production. This often means that such fatty acid feedstocks may contain free fatty acids and that their FFA contents can vary over a wide range.
Before such fatty acid feedstock is suitable for use in a transesterification process for biodiesel production the FFA content needs to be reduced to ≦0.25%. This may be performed by esterifying the FFA with a lower alcohol. However, the esterification reaction is reversible and leads to an equilibrium where one of the products formed by the esterification is water. Shifting the reaction equilibrium to the ester side therefore requires either the use of a large excess of lower alcohol or the removal of the formed water.
Accordingly, U.S. Pat. No. 4,164,506 discloses a process comprising the esterification of free fatty acids of unrefined fats with a lower alcohol in an amount larger than its solubility in the fats in the presence of an acid catalyst. However, several lower alcohols have a boiling point that is lower than the boiling point of water which implies that it is impossible to remove the water formed by the esterification while retaining the lower alcohol in the reaction mixture. Shifting the esterification equilibrium to the ester side therefore requires the use of a large excess of lower alcohol.
This disadvantage can be overcome by using a high boiling alcohol such as glycerol as disclosed in U.S. Pat. No. 2,588,435. Using such high boiling alcohols has the additional advantage that the reaction can be carried out at a higher temperature, which increases the rate constant of the esterification reaction, without having to operate under superatmospheric pressure. In fact, as disclosed in U.S. Pat. No. 6,822,105, the esterification can now be carried out under vacuum, which promotes the evaporation of the water formed by the esterification reaction which is thereby shifted towards the ester side. The use of nitrogen during a vacuum stripping operation further facilitates the water evaporation.
However, as demonstrated by the examples in U.S. Pat. Nos. 6,822,105 and 7,087,771, the esterification reaction is quite slow and it can take some 7 to 11 hours before the acid value of the reaction mixture, which is indicative of the residual free fatty acid content, has decreased to a value below 0.4 (mg KOH per g oil), which in industrial practice is the maximum value for a starting material for a transesterification process leading to biodiesel. The example in U.S. Patent Application Publication No. 2004/0186307 employing a solid esterification catalyst, which is present in a packed bed inside the esterification reactor, also mentions a reaction time of 5 hours at a temperature of 200° C. Holding fatty materials at such a high temperature for long periods of time can lead to the formation of unwanted side-products.
Accordingly, there is a strong preference for an esterification reaction at lower temperatures and for a catalyst that does not cause side-products to be formed. Operating at a lower temperature can also lead to energy savings. In this context, the use of enzymes in general and of lipases in particular merits consideration. However, the use of enzymes is far from straightforward. Their activity depends on the water concentration but water also affects the position of the esterification equilibrium. Moreover, the reagents should be well mixed, which is why the literature often mentions the use of solvents, e.g., Pastor et al., 1994, Applied Biochemistry and Biotechnology 50: 251-263: Synthesis of Mono- and Dioleylglycerols Using an Immobilized Lipase. For industrial processes the use of solvents raises the cost of operation and is therefore preferably avoided.
WO 2008/125574 discloses an esterification process wherein the formed water is removed from the reaction mixture by stripping with an inert gas. However, there is further need for enzymatic processes that allows fatty raw materials with variable free fatty acid contents to be utilized as raw material for biodiesel production.